This invention relates to a multi-mode (MMI) coupler, and more particularly to an MMI coupler having a polymer cladding.
In the processing of light beams for example, in telecommunications applications, important and desired functions are the splitting and combining of light beams. In conventional optics, prisms or pellicle splitters are used for this purpose. Attempts are continually being made to reduce the dimensions of the optical components to a considerable extent. On the one hand, it is being attempted in three dimensions to realise the processing of light beams by means of interference phenomena such as holography and free space optics. On the other hand, the technique of integrated optics is developing very rapidly. In this technique, waveguides are patterned on thin-film layers. It is an object of integrated optics to realise the functionality of the components used in conventional optics by new, integrable optical elements. This research field has found important applications in the field of communication.
In fiber-optical communication, data are transmitted by means of optical signals through otpical fibers. The optical signals are processed on integrated optical chips which are placed between the fibers. To manufacture these chips, generally thin-film layers are provided on support substrates such as, for example glass, Si, InP, GaAs and subsequently structured.
In optical waveguides the light is guided through a medium referred to as the waveguide core. The guidance is realised in that the waveguide core is bounded by a reflecting transition. In cavity waveguides, a metal is used for this purpose. In dielectric waveguides, the total reflection on a surrounding medium having a smaller refractive index the waveguide cladding is used. In optical waveguides, only those modes can propagate which fulfil the Maxwell equations. The waveguides are referred to as cut-off, monomode or multimode waveguides, dependent on whether they can guide no mode, only one mode for each polarization or a plurality of modes.
In waveguides, the light propagates in the longitudinal z direction. The x direction is parallel to the waveguide layer and is defined as the horizontal, or lateral direction. Analogously, the y direction is vertical to the waveguide layer and is defined as the vertical, or transversal direction. The propagation of light on the chips is computed by means of numerical methods such as beam propagation (BPM) methods, or modal propagation analysis (MPA) methods. In some cases, semi-analytical computations such as the effective index method (EIM) are used. The Maxwell equations are often solved in a scalar approximation. These equations describe the planar optics exactly. Two polarizations can be distinguished: the TE polarization has the E vector in the x direction. and the TM polarization has the H vector in the x direction. For the most frequently used dielectric waveguides in integrated optics, the scalar approximation leads to xe2x80x9cquasi-TExe2x80x9d and xe2x80x9cquasi-TMxe2x80x9d modes. In such waveguides, the mode forms and the xe2x80x9ceffectivexe2x80x9d indexes may be dependent on the polarization. In many cases it is very much desirable, but very difficult, to produce components which are independent of polarization. It should be noted that xe2x80x9cmonomodexe2x80x9d waveguides often have a mode for each polarization, i.e. overall, there are often two modes in xe2x80x9cmonomodexe2x80x9d waveguides. These are degenerate modes in normal optical glass fibers.
Important components in integrated optics are the beam splitters and combiners. Generally, one refers to Nxc3x97M splitters. Nxc3x97M denote the number of inputs and outputs. Ideally, these splitters should have the following properties: they should be compact (having small dimensions), independent of polarization, not very sensitive to manufacturing inaccuracies and easy to produce. Moreover, it should be possible to readily adapt the splitting or combining ratios to the various applications by geometrical changes in the design. Various beam splitters and combiners have already been realised: Symmetrical Y branches are simple solutions for 1xc3x972 splitters with a 50%/50% intensity ratio. Asymmetrical Y branches yield other intensity ratios but, due to coupling effects, they are often polarization-dependent between the two outputs. For manufacturing Y branches, a high resolution, particularly in the sharp bifurcation, is required. They are very sensitive to manufacturing inaccuracies.
Directional couplers with two parallel waveguides separated by means of a narrow slit operate as 2xc3x972 splitters. However the coupling length is, however, very sensitive to manufacturing parameters, particularly as regards slit width and depth. The coupling length is also very much dependent on polarization. xe2x80x9cTwo-modexe2x80x9d interference (TMI) couplers without a slit also operate as 2xc3x972 splitters. The intensity ratio is, however, very much dependent on the coupling into the input and output Y branches. Consequently, they are very sensitive to manufacturing conditions. U.S. Pat. No. 5,857,039 describes a thermo-optically activated directional coupler having a polymer guide buffer layer and a heater which allows the refractive index of the polymer to be varied. Of course it is well known that polymer has a higher refractive index variation with temperature than silica and better heat confinement. The ""039 patent extols the virtues of polymer over silica especially in the interguide space. When the interguide space is small in relation to the dimensions of the cross section, the guides can only be properly covered by the mineral layer, particularly in the case of silica coverings, by a so-called xe2x80x9cFHDxe2x80x9d technique which is extremely difficult to apply. Therefore, the ""039 patent provides a solution which is tailored to providing a more practicably directional coupler.
It is somewhat obvious, in hindsight, that in a directional coupler wherein coupling of light across a cladding boundary between two closely spaced waveguides is to be accomplished, that the boundary region must be controlled to increase or decrease the coupling across this region. Ergo, in order to allow the two single mode signals to couple, or to remain isolated, in a controlled manner, this intermediary cladding region must be highly manufacturable and controllable. As the ""039 patent purports, a polymer disposed between these cores, provides a practicable solution.
The use of a polymer cladding on an silica filament strand of optical fibre is well known, and has been disclosed in U.S. Pat. No. 4,116,654 issue Sep. 26, 1978. In this patent it is stated that xe2x80x9cWhere low attenuation of transmitted light in an optical fiber material is critical, the preferred material for the filamentary core is silica, since it has one of the lowest attenuations presently known. Suitable cladding materials known in the art include thermoplastic polymers which have an index of refraction lower than that of the core; and which preferably are substantially amorphous.xe2x80x9d
A further mention of polymer cladding is found in U.S. Pat. No. 5,873,923 in the name of DiGiovanni, with reference to optical amplifiers.
In this patent, a polymer cladding is suggested in a multi-clad fibre amplifier, where the 923 patent states that xe2x80x9cAny polymer cladding serves little purpose beyond guidingxe2x80x9d.
Considering the teaching of DiGiovanni, and that of U.S. Pat. No. 5,857,039, it is evident that the cladding guides the light within a waveguide or optical fibre and when the relative refractive index difference between the cladding and the core is varied the confinement of light within a guide varies as well.
What is surprising however, is that significance of providing a cladding on a multi-mode coupler which operates under a very different regime. What is further surprising is how coupling within a wide MMI core is affected by varying the cladding on top. Notwithstanding, this will be explained.
In the last few years, multimode interference (MMI) couplers have become more and more popular. These components are waveguide sections guiding a plurality of modes. They are produced, for example by widening a conventional waveguide structure until it guides a sufficient number of modes. The lateral guidance is then, for example, also often increased. Thanks to their self-imaging property, these couplers operate as Nxc3x97M splitters in two or three dimensions. xe2x80x9cConventionalxe2x80x9d MMI couplers as used throughout the specification and the following claims are understood to be those elements having parallel sides. It is to be noted that MMI couplers can also be made with slanting sides. Several prior art patents describe the function and operation of the MMI couplers, such as U.S. Pat. No. 5,698,597 in the name of Besse, issued Nov. 18, 1997, and U.S. Pat. No. 5,953,467 in the name of Madsen issued Sep. 14, 1999, both incorporated herein by reference. Since the invention deals exactly with this point, it is necessary to elucidate the properties of the xe2x80x9cconventionalxe2x80x9d MMI couplers.
Heretofore, MMI couplers have been very difficult to manufacture with a great deal of accuracy. Notwithstanding, since these devices are not highly tolerant to imperfect manufacturing, producing MMI couplers has been a feat, and until now, has remained a costly process. The invention discovered here concerns providing a polymer layer atop the MMI wide core and lessens the requirement for accuracy in manufacturing and allows a device to be tuned to meet required specifications, within predetermined limits. For example, a poorly manufactured device can be tuned to perform as a perfectly manufactured device that meets its original specifications. Hence, many fewer devices are rejected and discarded as rejects.
A second advantage of this invention is that devices can be tuned to provide a controlled and variable output. For example, the MMI coupler can function as a switch or a variable coupler.
In hindsight, after considering this invention, it is not intuitive, since multi-modes are mixed with a single wide guide having confinement only in the vertical dimension wherein the modes are xe2x80x9cessentiallyxe2x80x9d free to mix in a lateral dimension; thus it is quite surprising that the discovery of the provision of a polymer confining layer over top of the single multi-mode guide would yield any significant advantage.
Notwithstanding, best mode working embodiments will be described.
It is an object of the invention to provide an MMI coupler that is tunable and that is more tolerant to manufacturing inaccuracies due to its tunability.
In accordance with the invention a multi-mode interference coupler for coupling light between ports is provided, comprising:
a first input port for launching light into a core of a substantially planar waveguide of a first material having a first refractive index n1, the substantially planar waveguide having a response that confines the light to a single mode in one dimension, and multi-mode in another dimension;
at least two output ports for receiving light launched into and propagating through the core from the input port;
a polymer cladding of a second material having a refractive index n2 covering at least a portion of the core; and,
a heater thermally coupled to said cladding for heating the cladding in dependence upon a control signal, to vary at least one of
a) the amount of light received at the at least two output ports and,
b) the ratio of light distributed between the output ports,
wherein at least one of the first material and the second material is a polymer.